Questioning assumptions behind Bell's and related theorems

[DevilsAvocado]

The Emergent Copenhagen Interpretation of Quantum Mechanics
[...]
Finally we give an account of the EPR-Bohm thought experiment and show that the interpretation implies the violation of the Bell inequality characteristic of quantum mechanics but in a way that is rather novel.

http://arxiv.org/abs/1312.3427]The[/PLAIN] key point point is that the interaction between A and 1 changes the ontic states of 1 and 1 + 2 but not 2.

Hi DrC, is this really correct? If we are talking perfect (anti)correlations, then there’s no choice for 2, after interaction between A and 1 the state for 2 is determined, right...??

 

[bohm2]

It might seem a little bit 'rude', but to me it looks like Prof. Accardi is building a 'classical card castle' around the theoretical chameleons – with no or very little substance – and then to prove the whole thing he exploits the detection loophole (which is embarrassingly easy to do for Mermin’s 'counterfactual argument' in Bell’s theorem).

His argument has little to do with the detection loophole. You might want to look at these threads and particularly the debate between DrChinese and billschnieder:

https://www.physicsforums.com/showthread.php?t=496839&page=6
https://www.physicsforums.com/showthread.php?t=499002

I tried reading the Hand De Raedt articles but I could not follow the math; nevertheless, the discussion/debate between DrChinese and billschnieder was very informative.

 

[DevilsAvocado]

His argument has little to do with the detection loophole.

I don’t agree. He spends several pages on the “Difference between coincidences and efficiency of the detectors”. Why? If you have a mathematical theory that is correct and in accordance with the true nature of the world – why on Earth would you mix in human shortcomings in the measuring apparatus??

Answer: He builds the whole thing on tuning the efficiency to fit his theory.

http://arxiv.org/abs/quant-ph/0112067]Suppose[/URL] that a detector is 100% efficient. Then, if a source emits 100 photons, all photons are detected in absence of polarizer. Suppose moreover that, when the polarizer is inserted, only 90 photons and not 100 are detected. Therefore, if as done in [18], the efficiency is calibrated with the ratio of the number of particles detected by the detector with polarizer and without polarizer, we should conclude that our polarizer is 90% efficient.
However, if the loss of these 10 photons is due to the chameleon effect, then by repeating many times the experiment (and postulating a situation of stationarity of the source) one should always detect 90 photons.
On the contrary, if the loss of photons is due to accidental causes, then the number of detected photons should fluctuate and an analysis of these fluctuations should, in principle, allow to distinguish between an 100% efficient detector in presence of the chameleon effect and an 100% efficient detector in presence of a 90% efficient polarizer.
In real physical situations the two effects are most likely combined and their distinction, although clear in principle, might be a very hard challenge both for theoreticians and experimentalists. However we are convinced that a satisfactory theory of measurement should take into account both these effects.

I would say that if one is to construct something as peculiar as a “Theory of Measurement”, then one should not be too restricted in type of equipment... otherwise there’s quite a risk that someone will run you over with a new type of experiment that you just missed...

Experimental violation of a Bell's inequality with efficient detection

Here we have measured correlations in the classical properties of massive entangled particles (⁹Be⁺ ions): these correlations violate a form of Bell's inequality. Our measured value of the appropriate Bell's ‘signal’ is 2.25 ± 0.03, whereas a value of 2 is the maximum allowed by local realistic theories of nature. In contrast to previous measurements with massive particles, this violation of Bell's inequality was obtained by use of a complete set of measurements. Moreover, the high detection efficiency of our apparatus eliminates the so-called ‘detection’ loophole.

You might want to look at these threads and particularly the debate between DrChinese and billschnieder:

OMG... I get the creeps just thinking about these “do-nuts” and “French doctors” performing cruel allergy tests on patients...

Okay, I’ll give it a (hopefully) last try...

It’s seems very popular among the “LR-gang” to come down hard on Bell for identifying Local Realism inaccurate (maybe because it’s their last hope). The truth is that Einstein-causality (locality) of course comes from Einstein, and so does also realism. In correspondence with Max Born (March 1948), he writes:

“That which really exists in B should …not depend on what kind of measurement is carried out in part of space A; it should also be independent of whether or not any measurement at all is carried out in space A. If one adheres to this program, one can hardly consider the quantum-theoretical description as a complete representation of the physically real. If one tries to do so in spite of this, one has to assume that the physically real in B suffers a sudden change as a result of a measurement in A. My instinct for physics bristles at this.”

I don’t know... (maybe I’m unfair), but I get a slight feeling that this 'confusion' (on PF) regarding definite properties, somehow emanate from DrChinese’s “Bell's Theorem with Easy Math”... (truly sorry DrC if this is wrong!), and the mathematical abstraction he introduce to make things a little bit easier. We have all seen billschnieder go do-nuts over 3 vs 1 measured property, and some are fatally convinced that it’s a “magician's trick”, like trying to fit a circle onto a square... impossible from the beginning and hence unfounded as a theorem.

I think this unfortunate situation is due to that DrChinese (and I’m writing this to assist, not oppose DrC) in his example uses only the combinations [AB] [BC] [AC] and never [AA] [BB] [CC]. The later proves (without any discussion on the validity of unmeasured properties) that the LHV has to be prepared for perfect correlations, i.e. they have to agree – at the source – on what to do in case of perfect correlations. Here the only options are ++ or -- and there’s no statistical dependency in this case. One pair has to agree on this value on beforehand, it cannot differ in any single measurement.

This means that the LHV must have a predefined property/value/function/chameleon, or whatever, for all 3 cases of [AA] [BB] [CC] since there is no way to know in advance what will happen at the measuring apparatus (unless you have non-local function finding this out for you).

This shows without doubt and with – empirical verifications – that statements like this:

"The mistake here is that Bell and followers insist from the start that the same element of reality occurs for the three different experiments with three different setting pairs."​

is catastrophically wrong. Perfect correlations in LHV demands three predetermined values, for three different settings, in a single measurement/experiment. Period.

If we take one step back to Einstein/EPR, things hopefully becomes even clearer. They used definite position and definite momentum as criteria instead of definite photon polarization. I guess everybody understands how hilarious things become if one simultaneous is claiming that local realism is indeed still alive, but it’s terribly wrong to state that a particle in this world will have a definite position and a definite momentum at all times!

For God’s sake – where is the particle in this new weird interpretation of local (sur)realism!?

This is nothing but the extended version of One Flew Over the Cuckoo's Nest II (Director's cut).

Of course we can also translate the perfect correlations to billschnieder’s infamous “do-nuts” setup:

Bill and his twin brother Buffalo are the main characters in this experiment. Buffalo is stationed in L.A. and Bill in NYC. They hired a baker in Omaha NE to send them each three black boxes of goodies, every morning, with the LR-express.

The three black boxes contain “do-nuts”, “fruitcake” and “half-baked potatoes”, separately.

Buffalo & Bill eagerly want to demonstrate how easy it is to mimic the dreadful perfect correlations in QM, in a completely local and very realistic way. So they create a set of rules:

1) They can only open one box a day.
2) If current date is even, they shall eat what’s inside the box.
3) If current date is odd, they shall give what’s inside the box to their fat cat.
4) They must throw the unopened boxes in the garbage every night.
5) They are not allowed to communicate, in case of “black box confusion”.

This very interesting experiment goes on for a month and then Buffalo travel to NYC to compare the data. Amazingly enough the data matches perfect correlations in QM, if we only consider the days when Buffalo & Bill happened to get the same kind of goodies!​

From this little fairytale we can draw the conclusion that if the baker in Omaha had put vacuum in the black boxes or dough to be baked on site – this experiment would have failed completely.

I surely hope billschnieder realize this as well...

And from this we can very easily see that if Buffalo & Bill want to go further and also mimic the rest of the statistics of QM, they run their heads into the solid concrete wall of elementary mathematics, which DrChinese has demonstrated so well in his example.

It’s mathematical impossible!

I tried reading the Hand De Raedt articles but I could not follow the math;

Neither do I, but I do understand some parts in his software and it does not convince me in any way.

 

[zonde]

This means that the LHV must have a predefined property/value/function/chameleon, or whatever, for all 3 cases of [AA] [BB] [CC] since there is no way to know in advance what will happen at the measuring apparatus (unless you have non-local function finding this out for you).

This shows without doubt and with – empirical verifications – that statements like this:

"The mistake here is that Bell and followers insist from the start that the same element of reality occurs for the three different experiments with three different setting pairs."​

is catastrophically wrong. Perfect correlations in LHV demands three predetermined values, for three different settings, in a single measurement/experiment. Period.

There are no dedicated tests of perfect correlations. It still remains untested prediction of QM with some experimental observations that this might be true - I am commenting your claim about empirical verifications.

Even so we have practically 100% efficient photon detectors they are used for attempts at falsification of "local realism" rather than attempts at falsification of QM.

 

[zonde]

It is mainstream that either entangled particle spin is not predetermined, or there are non-local factors affecting it.

I would challenge that "either" part of your statement.
How a local measurement of particle spin that is not predetermined can produce non-local correlations? There should be sources explaining this if it's mainstream view.

 

[morrobay]

It is mainstream that either entangled particle spin is not predetermined, or there are non-local factors affecting it.

Can the interpretation of the above be that it is mainstream that entangled particle spins are not predetermined and non-local factors affect second measurement ? Ie, A measurement of an entangled particle at detector A that is in superposition , spin up,spin down is made on z axis. It is spin up. The second measurement is made at space-like separated detector B on same axis and is anti correlated , spin down.
Non local effect.

 

[Nugatory]

Can the interpretation of the above be that it is mainstream that entangled particle spins are not predetermined and non-local factors affect second measurement ? Ie, A measurement of an entangled particle at detector A that is in superposition , spin up,spin down is made on z axis. It is spin up. The second measurement is made at space-like separated detector B on same axis and is anti correlated , spin down.
Non local effect.

If the two measurements are space-like separated, then there is no "first" or "second" measurement - their ordering is different for observers moving at different relative speeds because ofthe relativity of simultaneity.

 

[morrobay]

If the two measurements are space-like separated, then there is no "first" or "second" measurement - their ordering is different for observers moving at different relative speeds because ofthe relativity of simultaneity.

Space-like that the measurement at detector A, a distance (d) from detector B, is made at t₁ and the measurement at detector B is made at t₂ such that t₂ < d/c

 

[Nugatory]

Space-like that the measurement at detector A, a distance (d) from detector B, is made at t₁ and the measurement at detector B is made at t₂ such that t₂ < d/c

If $\Delta{t}<\frac{d}{c}$ for any observer, then for some observers $t_1 < t_2$ while for other observers $t_2 < t_1$ - so which one is "first"?

 

[DevilsAvocado]

Can the interpretation of the above be that it is mainstream that entangled particle spins are not predetermined and non-local factors affect second measurement ? Ie, A measurement of an entangled particle at detector A that is in superposition , spin up,spin down is made on z axis. It is spin up. The second measurement is made at space-like separated detector B on same axis and is anti correlated , spin down.
Non local effect.

Besides Nugatory’s correct objection, it will not work because A & B are only perfectly (anti)correlated on (anti)parallel settings (i.e. the old EPR picture). In all other cases there are statistical correlations ranging from 0.01 to 0.99. This is Bell's ingenious contribution to EPR that finally settled the Bohr–Einstein debates.

Bell’s theorem stipulates that QM violates at least one of these three assumptions:

• Realism
• Locality
• Free will

 

[DevilsAvocado]

I would challenge that "either" part of your statement.
How a local measurement of particle spin that is not predetermined can produce non-local correlations? There should be sources explaining this if it's mainstream view.

Entanglement and the shared wave function, described by Erwin Schrödinger 1935:

http://www.tuhh.de/rzt/rzt/it/QM/cat.html]THE[/PLAIN] PRESENT SITUATION IN QUANTUM MECHANICS

The remarkable theory of measurement, the apparent jumping around of the psi-function, and finally the "antinomies of entanglement", all derive from the simple manner in which the calculation methods of quantum mechanics allow two separated systems conceptually to be combined together into a single one; for which the methods seem plainly predestined. When two systems interact, their psi-functions, as we have seen, do not come into interaction but rather they immediately cease to exist and a single one, for the combined system, takes their place.

 

[stevendaryl]

It is mainstream that either entangled particle spin is not predetermined, or there are non-local factors affecting it.

That statement to me is weaker than it needs to be. That was of phrasing things would seem to leave open the possibility of a local, non-deterministic hidden-variables theory (where spin results are not predetermined but are instead the result of a stochastic process). But there is no such local stochastic model of EPR-type correlations.

 

[DrChinese]

That statement to me is weaker than it needs to be. That was of phrasing things would seem to leave open the possibility of a local, non-deterministic hidden-variables theory (where spin results are not predetermined but are instead the result of a stochastic process). But there is no such local stochastic model of EPR-type correlations.

Perhaps. I would say that there can be no such local stochastic model because of perfect correlations. Such a model would inevitably feature something which insures a specific outcome (since the settings/detectors themselves cannot introduce any element of randomness). Ergo it must be predetermined from hidden variables.

 

[zonde]

Entanglement and the shared wave function, described by Erwin Schrödinger 1935:

This quote does not answer my question. My question was how you do away with spooky action at a distance by assuming that measurements are just random.

Besides it does not seem like Erwin Schrödinger is explaining solution but rather a problem in that paragraph. After all the paragraph ends with this sentence: "Best possible knowledge of a whole does not include best possible knowledge of its parts - and that is what keeps coming back to haunt us."

Bell’s theorem stipulates that QM violates at least one of these three assumptions:

• Realism
• Locality
• Free will

Realism and free will are very basic assumptions behind scientific method. You could simply state that QM is either non-scientific or it violates locality IMHO.

 

[morrobay]

If $\Delta{t}<\frac{d}{c}$ for any observer, then for some observers $t_1 < t_2$ while for other observers $t_2 < t_1$ - so which one is "first"?

Let me re state and clarify to show that measurement at t₁ at detector A is first. And a non - local effect in second measurement , t₂ at detector B : Suppose both detectors are in same frame or in comoving frames.
The source of the entangled photons is 9/20 of total distance (d) from A and 11/20 of total distance (d) from B on AB axis The first measurement at t₁ = 9/20 d/c The particle before measurement was in superposition spin up spin down on a parallel setting. It is spin up, collapsed wave function.
The second measurement t₂ at detector B, same setting , equals 11/20 d/c. Spin down, anti correlated. So the second measurement t₂ was 1/10 d/c after first measurement. Then Δt₂ < d/c (superluminal signal between t₁ and t₂). Again my interpretation of mainstream view, non predetermined spin values for entangled particles and non locality. .

 

[DevilsAvocado]

This quote does not answer my question.

Oops sorry, bad interpretation on my side.

My question was how you do away with spooky action at a distance by assuming that measurements are just random.

DrC can speak for himself, but to me it looks like he is talking about that the only way to have determined entangled particle spin, is thru some non-local function, i.e. de Broglie-Bohm theory.

On the other hand, if we interpret your way; i.e. what happens in EPR-Bell if we keep locality and exclude realism (in the “Three Amigos” above)?

That’s a good question, and the most important answer is that Bell’s theorem is a no-go theorem – i.e. it states that that a particular situation is not physically possible. In Bell’s theorem this impossible situation is LHV compatible with the predictions of QM.

This means that to refute Bell’s theorem you must also prove QM wrong.

This will, with no doubt, be the biggest task in history of science (certainly not compatible with the rules of this forum), and if this (against all odds in the observable universe) will ever happen – your computer and every other electronic gadget on this planet will stop working in a fraction of a second, and we will be thrown back to the dark ages of nineteenth century, using telegraphs and steam engines.

Back to your question; Are there any attempts to explain how locality & non-realism would work?

Yes there are, where non-realism is implemented by means of nonseparability, in for example a relational blockworld.

Realism and free will are very basic assumptions behind scientific method. You could simply state that QM is either non-scientific or it violates locality IMHO.

I think Lawrence Krauss said – "the universe isn't designed for us" – and QM has proven that this is at least true regarding the human brain.

You could easily replace QM for My computer, and say; “My computer is either non-scientific or it violates locality IMHO”.

What fits best in the mouth, I leave to you and any other reader out there...

 

[stevendaryl]

This quote does not answer my question. My question was how you do away with spooky action at a distance by assuming that measurements are just random.

Besides it does not seem like Erwin Schrödinger is explaining solution but rather a problem in that paragraph. After all the paragraph ends with this sentence: "Best possible knowledge of a whole does not include best possible knowledge of its parts - and that is what keeps coming back to haunt us."

Realism and free will are very basic assumptions behind scientific method. You could simply state that QM is either non-scientific or it violates locality IMHO.

Personally, I find the terms "free will" and "realism" to be too fuzzy to reason about. I think it's more useful to think in terms of possible models that are ruled out by invoking "free will" or "realism".

As for "free will", a sort of model that is local, deterministic and compatible with the QM prediction is a "superdeterministic" model. In the reasoning that leads up to Bell's inequality, it is assumed that the choice of the hidden variable is independent of the choice of settings of distant measurement devices. That might not be the case. If the world is deterministic, then the settings of detectors is determined long in the past, and so it is possible to choose the hidden variable in a way that takes into account the future settings. (Actually, there's an interesting--to me--question about whether superdeterminism requires that twin-pair sources and detectors have an overlap in their backward lightcones. In some GR cosmologies, that might not be the case for really distant detectors.) I think it would be really difficult to reason about such a superdeterministic theory, but not impossible.

I don't really know what people mean by "realism". What is an example of a non-realistic theory? Well, I suppose one could explain the EPR correlations by assuming that the world is just a dream.

 

[stevendaryl]

DrC can speak for himself, but to me it looks like he is talking about that the only way to have determined entangled particle spin, is thru some non-local function, i.e. de Broglie-Bohm theory.

The problem I have with Dr. C's either/or is that I don't see how a nondeterministic local realistic theory can reproduce the predictions of QM, either. So what's ruled out is local realistic models, and determinism is irrelevant.

 

[Nugatory]

Let me re state and clarify to show that measurement at t₁ at detector A is first. And a non - local effect in second measurement , t₂ at detector B : Suppose both detectors are in same frame or in comoving frames.
The source of the entangled photons is 9/20 of total distance (d) from A and 11/20 of total distance (d) from B on AB axis

And therefore you, at rest relative to the source and both detectors, are quite clear that $t_1 < t_2$ and that the detection at A happens before the detection at B. I, however, am watching from a spaceship moving along the AB axis at a speed of .1c relative to your lab and I will observe that $t_1 = t_2$ so the two detections were simultaneous. At any greater speed, I would find that $t_1 > t_2$ so the detection at B came first.

(Someone check my math, please - I did the algebra in my head so .1c may not be right for this particular combination of distances).

This is a typical example of the relativity of simultaneity at work; there is no way of deciding which of two spacelike-separated events "really" happened first. It's also why the apparent faster-than-light propagation of entanglement effects is so perplexing.

 

[DevilsAvocado]

The problem I have with Dr. C's either/or is that I don't see how a nondeterministic local realistic theory can reproduce the predictions of QM, either. So what's ruled out is local realistic models, and determinism is irrelevant.

Interesting, I have never thought of this... nondeterministic local realistic theory... is that even possible? Let's see:

1. The entangled particles are in a superposition of correlated random outcomes.
2. Non-locality is not present to assist.
3. Realism is a fact.

But... afaik, realism requires that "the Moon is there even when nobody looks"... which means the particles assembling the Moon must have definite states all the time, i.e. superposition is out of the question. If this also excludes "classical randomness", I don't know... how would a "stochastic Moon" look like??

AFAIK, Einstein spent a lot of time debating randomness vs realism, and I don't dare to enter these deep waters, but this setup will have serious problems just combining 1 & 2, which means it's not compatible with the predictions of QM.

I think many do put an equal sign between determinism and realism so when realism goes, so does determinism. We can at least be sure that if we exclude non-locality, determinism is the only way to get perfect correlations in EPR-Bell, which then is refuted by the full predictions of QM (i.e. all the other correlations/settings).

 

[DrChinese]

The problem I have with Dr. C's either/or is that I don't see how a nondeterministic local realistic theory can reproduce the predictions of QM, either. So what's ruled out is local realistic models, and determinism is irrelevant.

If one of Bell's assumptions (such as deterministic hidden variables) is wrong, then there could be a nondeterministic local model that can reproduce the predictions of QM. I don't claim it to be realistic, however, so I don't follow that part.

You are asking "how is that possible physically?" or similar (what is the mechanism etc). I can't say I know any more than I can answer a lot of "how" questions. I choose (and this is consistent with Bell) to think of such a model in terms of time symmetry. The full experimental setup includes future variables (which include information about Alice and Bob's choice of measurement). I wouldn't speculate as to whether there is a root cause to the outcome of a quantum spin flip. And since there is still an element of randomness in the outcome, this is a non-deterministic model. It is also local because nothing is happening faster than c.

 

[DrChinese]

I think many do put an equal sign between determinism and realism so when realism goes, so does determinism.

That is how I see it too. I know there are others who draw a distinction between determinism and realism. That there is no distinction is best seen simply by referring to the EPR definition, which is what was used by Bell:

a) An element of reality exists if a prediction can be made with certainty.
b) Elements of reality do not need to be simultaneously demonstrable.

If you then define both realism and determinism around that, they must be the same thing for purposes of the EPR/Bell/Aspect line of reasoning.

 

[DrChinese]

And in my post above, I would add that requirement b) goes directly against QM's HUP. Therefore it is the weak link. No b) means Bell's realism assumption is invalid. And Bell's Theorem is satisfied because: No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics. You could also say: No physical theory featuring local pre-determination can ever reproduce all of the predictions of quantum mechanics. Or: No EPR-like physical theory can ever reproduce all of the predictions of quantum mechanics.

 

[stevendaryl]

If one of Bell's assumptions (such as deterministic hidden variables) is wrong, then there could be a nondeterministic local model that can reproduce the predictions of QM.

What I'm saying is that Bell's argument goes through perfectly well without assuming determinism. The assumption of deterministic hidden variables is that there is a hidden variable $\lambda$ that determines the outcomes at both detectors:

$F(\hat{A}, \lambda) =$ the outcome at Alice's detector when the hidden variable has value $\lambda$ and Alice's detector is at orientation $\hat{A}$

$G(\hat{B}, \lambda) =$ the outcome at Bob's detector when the hidden variable has value $\lambda$ and Bob's detector is at orientation $\hat{B}$

But if you started with a more general assumption (not assuming determinism), then there would be additional probabilities involved:

$P_A(\hat{A}, \lambda) =$ the probability that Alice will measure spin-up at orientation $\hat{A}$ when the hidden variable has value $\lambda$

$P_B(\hat{B}, \lambda) =$ the probability that Bob will measure spin-up at orientation $\hat{B}$ when the hidden variable has value $\lambda$

The perfect correlations that occur when $\hat{A} = \pm \hat{B}$ imply that
$P_B(\hat{B}, \lambda) =$ 0 or 1
$P_B(\hat{A}, \lambda) =$ 0 or 1

So determinism is a consequence of the assumption of local realism, not an additional assumption.

 

[audioloop]

Personally, I find the terms "free will" and "realism" to be too fuzzy to reason about. I think it's more useful to think in terms of possible models that are ruled out by invoking "free will" or "realism".


I don't really know what people mean by "realism". What is an example of a non-realistic theory? Well, I suppose one could explain the EPR correlations by assuming that the world is just a dream.

counterfactual definiteness, absence of definite values of quantum objects or process.
just that, but some wish more, i.e no existence, absence of anything.

but how can someone make science without anything ?

.

 

[stevendaryl]

But... afaik, realism requires that “the Moon is there even when nobody looks”... which means the particles assembling the Moon must have definite states all the time, i.e. superposition is out of the question. If this also excludes “classical randomness”, I don’t know... how would a “stochastic Moon” look like??

I don't see how being in a definite state at each moment in time implies that there is a unique future state.

 

[DrChinese]

So determinism is a consequence of the assumption of local realism, not an additional assumption.

I never said otherwise. I have said repeatedly that I use those terms interchangeably, that it is the EPR definition I follow (as does Bell). Realism = simultaneous elements of reality. Those determine measurement outcomes, there being no random element.

And I don't believe we live in such a realistic universe.

 

[stevendaryl]

counterfactual definiteness

I don't see that counterfactual definiteness is that important, conceptually. If you take classical mechanics and add a random number generator, you still have a theory that's not all that mysterious. You can still make predictions and do experiments, etc. It might be weird (and Einstein may not have liked such a theory) but it's not anywhere near as weird as QM. I wouldn't call such a theory "non-realistic". It's just nondeterministic.

 

[DevilsAvocado]

This is a typical example of the relativity of simultaneity at work; there is no way of deciding which of two spacelike-separated events "really" happened first. It's also why the apparent faster-than-light propagation of entanglement effects is so perplexing.

Agreed 100% :thumbs:

This was also a big dilemma for John Bell, who just a few month before he died, said that "this doesn't work, or it's just me, being silly", i.e. he was even prepared to dump his theorem as silliness, facing the problem of SR vs QM.

Many seem to sweep this question under the rug, saying "Well, there is NO usable information transmitted, therefore this is not an issue" or "What issue?? I have rigorously shown that with this equation, in the 11^th dimension, everything works as expected!".

Still we have correlations between the entangled particles, which shows that they 'knows' what happened to the other, even if the outcome (as always in QM) is random. And the outcome is definitely not in any 11^th dimension, it's right in front of our nose.

To me, the sign saying "STOP! VERY INTERESTING!" can't possibly be any larger than this...

 

[DrChinese]

This is a typical example of the relativity of simultaneity at work; there is no way of deciding which of two spacelike-separated events "really" happened first. It's also why the apparent faster-than-light propagation of entanglement effects is so perplexing.

I would disagree that "relativity of simultaneity" has anything to do with the perplexing ftl effects. You can see that quantum ordering is irrelevant in many cases in which ordering is absolute. An example is entanglement swapping such as this:

http://arxiv.org/abs/quant-ph/0201134

The decision to swap can be made anytime: before, after or between detection of pair members. This is true in any reference frame, relative or absolute. The results never vary.

 

[DevilsAvocado]

You could also say: No physical theory featuring local pre-determination can ever reproduce all of the predictions of quantum mechanics.

This is very nice, says it all. And in case of objections regarding pre-determination, all you have to say is "perfect correlations", which is then disputed by 'ordinary' correlations. Sweet!

 

[billschnieder]

And therefore you, at rest relative to the source and both detectors, are quite clear that $t_1 < t_2$ and that the detection at A happens before the detection at B. I, however, am watching from a spaceship moving along the AB axis at a speed of .1c relative to your lab and I will observe that $t_1 = t_2$ so the two detections were simultaneous. At any greater speed, I would find that $t_1 > t_2$ so the detection at B came first.

(Someone check my math, please - I did the algebra in my head so .1c may not be right for this particular combination of distances).

This is a typical example of the relativity of simultaneity at work; there is no way of deciding which of two spacelike-separated events "really" happened first. It's also why the apparent faster-than-light propagation of entanglement effects is so perplexing.

This is a red herring. If Alice an Bob have synchronized clocks, as they should/do in any such experiment, then according to their clocks (not any third-party) it makes sense to talk of a before/after/simultaneous. You could even eliminate clocks and talk of time as measured by the entangled pair themselves (e.g. distance traveled at c) and still it would make sense to talk of before/after/simultaneous. In EPR experiments, each station records the time of each detection event using synchronized clocks. The third-party analyzing the results uses those timestamps, not their own, so this issue you talk about is irrelevant.

You could change the before-/after-/simultaneous-"ness" by changing the path length traveled by the particles to get to each station and you will still obtain the correlations; so the question of direction of communication remains for those who hold to non-locality.

 

[DevilsAvocado]

I don't see how being in a definite state at each moment in time implies that there is a unique future state.

Maybe you’re right, but I think it will be hard to combine indefinite states with unique future states... i.e. indefinite states could hardly be a part of “classical realismâ€... and then we’re back to Einstein’s “classical randomnessâ€, which I interpret as “definite states that are real but not predictableâ€, and if this interpretation is correct, it is excluded by Bell’s theorem – if, and only if – locality is also required.

And this is due to definiteness, not randomness.

 

[billschnieder]

If one of Bell's assumptions (such as deterministic hidden variables) is wrong, then there could be a nondeterministic local model that can reproduce the predictions of QM. I don't claim it to be realistic, however, so I don't follow that part.

The paper which started this thread PROVED (without any doubt), that only two assumptions are required to obtain Bell's inequalities

(i) that the random variables take values in the interval [−1;+1]
(ii) that the random variables are de ned on the same probability space

Notice the absence of "determinism, or realism, or locality in those assumptions?

Bell's implicit assumption of the single probability space is equivalent to the postulate that, for each j = 1...N
pjI = pjII (3)
Physically this means that:
the hidden parameter in the fi rst experiment is the same as the hidden parameter in
the second experiment. This is surely a very implausible assumption.
Notice however that, without this assumption, Bell's argument cannot be carried over
and we cannot deduce the inequality because we must stop at equation (3).

 

[stevendaryl]

I never said otherwise. I have said repeatedly that I use those terms interchangeably.

Those terms being "determinism" and "local realism"? I wouldn't say that they are interchangeable. It just happens to be that for EPR correlations, there is no difference between the two.